The field of the present invention relates to an apparatus and method for machining pistons for positive displacement machines. In particular, the present invention relates to an apparatus and method for machining pistons for internal combustion engines, especially pistons for engines with low volume requirements.
A piston of an internal combustion engine includes a piston head having a top ring groove and an oil ring groove. A skirt is formed under the piston head. The skirt is connected to a connecting rod via a piston pin which is perpendicular to the center axis of the piston.
The piston executes a back and forth motion in the engine cylinder due to the combustion pressure force Fg of a combustion chamber formed in the cylinder above the piston head, and thereby rotates a crankshaft situated at a lower position, which is connected via the piston pin and connecting rod.
As the piston moves from top dead center to bottom dead center, the combustion pressure Fg applies a force Fc in an axial direction to the connection rod via the piston. A force Ft simultaneously acts on the piston in a horizontal direction according to the inclination angle of the connecting rod, and the skirt of the piston is pressed by this horizontal force Ft against the wall of the cylinder on a first side of the cylinder. When the piston rises from bottom dead center, the inertia of the crankshaft pushes the piston upward via the connecting rod, and the skirt of the piston is thereby pressed against the wall of the cylinder on the second side of the cylinder opposite the first side.
The piston skirt has the function of supporting the piston such that the latter does not lean due to the combustion energy acting upon it. The skirt tends to suffer more thermal expansion in a perpendicular direction to the piston pin due to the combustion heat of the engine. Therefore, it is typically formed with an elliptical cross-section having the piston pin as its short axis and the perpendicular direction as its long axis. The requirement of ovality in the machining of the piston skirt is one of the major challenges in the fabrication of pistons.
Although most vehicle manufacturers offer a large variety of vehicle and vehicle body styles, the types of engines offered by any vehicle manufacturer are extremely limited and tooling for building an engine typically will have a production run of seven years to a decade or more. In very high volumes, it is most practical when machining a piston to provide a separate station for virtually each operation.
Therefore, there will be one station to rough finish the side surface of the piston. There will be another station or stations to do the various machining operations upon the piston heads. There will be another station or stations to put in the top ring groove and the oil ring groove. There will be yet another station to provide the ovality to the skirt and still other stations to provide the bore for the piston pin.
Even minor changes in piston dimension and/or material can often require major changes in tooling. Therefore, changes to the piston design are avoided as much as possible.
The above-noted multi station method of machining pistons in high volume is suitable for large production runs. However, the multi station method of machining pistons is not advantageous when machining replacement parts where the production runs may not be in the hundreds of thousands or millions of pistons. Often, design changes which would lead to better functionality of the piston are avoided due to the capital costs incurred in the multi station method.
Another problem of the multi station method is that as the piston is moved from station to station, the piston work piece must be refixtured. Accuracy and/or time must be sacrificed to insure that the refixturing of the piston work piece is achieved correctly.
As previously mentioned, pistons have a top ring groove. The top ring groove seats a sealing ring. It is desirable for the top ring groove to be as close as possible to the end of the piston head. The further down the top ring groove is, the greater the amount of combustion volume wasted for a given piston cylinder combination.
A limiting factor of the placement of the top ring groove is the strength of the aluminum metal above the top ring groove and an extreme top end of the piston head. An increase in the strength of the piston would allow the portion of the piston head above the top ring groove to be lessened and therefore provide a more efficient piston.
To lower the amount of energy consumed by the engine and the reciprocal motion of the pistons, the pistons are typically made of a light metal such as aluminum. To increase the strength of the piston above top ring groove and also to lower the weight of the piston, certain composite products comprising an aluminum metal matrix and a strengthening or reinforcing phase such as ceramic particulars, whiskers, fibers or the like, have been brought forth. The composite pistons combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of a metal matrix. The fibers, ceramics, or carbide inserts which are added to the aluminum increase the strength and/or lower the weight of the piston.
Lowering the weight of the piston by the use of composite products also increases the gas mileage of the vehicle the piston is utilized. Composites also can deform less than pure aluminum and can bring about better deformation characteristics. However, the materials utilized in the making of composite pistons can often snag a cutting tool when the piston is being machined. Therefore, the application of composite pistons has been limited due to the machining constraints.
It is desirable to provide an apparatus and method to economically machine small volume runs of pistons. It is desirable to economically machine composite pistons which have been reinforced with fiber ceramics or carbides. It is also desirable to provide an apparatus and method to economically machine pistons that can allow for design changes in the pistons without incurring substantial capital costs for retooling.
It is still further desirable to provide an economical apparatus and method for high volume piston machining wherein the major machine operations which take place on the piston are performed without refixturing the piston for different operations. Elimination or a reduction in refixturing pistons in the manufacturing process can greatly reduce the scrape rate experienced in the manufacturing of pistons.
To make manifest the above noted and other unfulfilled desires, a revelation of the present invention is brought forth. In a preferred environment, the present invention provides a method of machining a piston. The piston work piece is presented to a machining center in the form of a rough casting having an inner cavity. The piston is fixtured by grabbing the piston along the inner cavity. The piston is oriented in a vertical manner and rotated while a mill cutter rough cuts the side surface of a head and skirt of the piston. The finish cutting of the skirt of the piston is performed by an ovality unit with a cutter head which is translationally mounted with respect to the piston and moves toward and away from the center line of the piston as the piston is rotated along its longitudinal axis.
In cooperation with a multiple tool turret miller, the piston is rotated and the piston ring grooves are cut into the side surface of the piston. Optionally, the milling on the top head of the piston is also performed. The piston is then held stationary and a drill is utilized to cross-bore the piston to make the pin connection. The piston is then released with all or virtually all of the machining complete.
It is an object of the present invention to provide an apparatus and method for machining a piston without requiring that the piston be refixtured. It is an object of the present invention to provide an apparatus and method of machining a piston which is economical for pistons having small production runs. It is an object of the present invention to provide an apparatus and method of machining a piston wherein the design changes can quickly and easily be made without the requirement of extensive retooling.
The above noted and other objects of the present invention will become apparent to those skilled in the art from a review of the invention as it is provided in the accompanying drawings and detailed description of the preferred embodiment.